Circuit arrangement for protecting an amplifying tube against overloading



Sept. 11, 1951 Filed June 24, 1947 INPUT AMPLIFIER INPUT AMPLIFlE/Z BUY CIRCUIT-ARRANGEMENT FOR PROTECTING AMPLIFYING INVENTOR. PETER KLA55 BUYAS.

AGENT.

TUBES AGAINST OVERLOADING 2 Sheets-Sheet 1 Sept. 11, 195 P K 2,567,272

1 BUYS CIRCUIT-ARRANGEMENT FOR PROTECTING AMPLIFYING TUBES AGAINST OVERLOADING Filed June 24, 1947 2 Sheets -Sheet 2 INPUT AMPLIFIER j 121:: 5 INPUT J AMPL lF/ER INVENTOR. PIETER TKLASS BUYS.

A GEL VI Patented Sept. 11, 1951 CIRCUIT ARRANGEMENT FOR PROTECTING AN AMPLIFYING TUBE AGAINST OVER- LOADING Pieter Klaas Buys, Eindhoven, Netherlands, as-

signor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application June 24, 1947, Serial No. 756,730 In the Netherlands June 27, 1946 8 Claims.

This invention relates to a circuit-arrangement for protecting an amplifying tube, which is loaded by an impedance in class AB-, 3- or C-setting, from overloading and, more particularly, of pushpull connected final tubes in class B-setting. As is well known with amplifying tubes in any of the aforesaidsettings there is a risk of overloading of the tubes if the load impedance is too low.

This applies, for instance, to amplifiers for radio-relay exchanges, in which the-load of the amplifier varies in accordance with the number of listening subscribers or in the case of shortcircuits.

In order to obviate overloading provision is made according to the invention, for controlling the effective input voltage in accordance with the value of the load impedance. This control is governed by the difference between a voltage takenfrom the input circuit of the tube to be protected and from the input circuit of an amplification stage preceding this tube respectively, and a Voltage taken from the output circuit of the said tube. I

in thepre'sent case the term contro' should be understood to mean control in the widest sense that i's'to say any control as a result of which the effective voltage supplied to the grids of these tubes (i. e. the voltage which occurs during that part of the cycle of the alternating grid voltage during which anode current passes through the tube) has a smaller amplitude than it would be the case without control.

. In order that the. invention may be clearly understood and carried into effect, it will now be explained more fully with reference to the accompanying drawings in which several embodiments thereof are represented by way of example.

In the embodiment shown in Fig. 1 a voltage proportional to the output voltage is applied to the first grid, and a voltage proportional to the input is applied to the second grid of an auxiliary tube. The two voltages are in phase-opposition and their value is such that, if the final tubes are adequately loaded, the auxiliary tube does not carry anode current, even if the voltage taken from the output increases. It does, however, if this voltage decreases. The anode current may be rectified and may influence the negative grid bias and consequently the mutual conductance of the final tubes and /or of one or more preceding amplifying tubes, as a result of which the effective input voltage of the final tubes decreases.

In the embodiment shown in Fig. 3, the circuitarrangement of which is similar to that in Fig. 1,

the effective voltage applied to the grids of final tubes l and 2 is reduced due to a voltage obtainedfrom the anode circuit of two auxiliary tubes I I and I I.

In a variant of the two embodiments shown in Fig. 4 a rectified portion of the output voltage influences, according to the invention, the mutual conductance of an auxiliary tube I I, the input voltage being fed to a control-grid of this tube.

The embodiment shown in Fig. 5 is different from that shown in Fig. 1 inasmuch as one of the voltages is operative in the cathode circuit,,

the other is the grid circuit of the auxiliary tube I I.

The circuit-arrangements shown in Figs. 1, 3, 4 and 5 will now be explained more fully.

The input voltage is fed through terminal 3, 4 to the control-grids of the two final tubes I and 2 operating in class B-setting (Fig. 1). The anode circuit of these tubes comprises an output transformer 5, the secondary part of which is loaded by loads which are connected to terminals 6, 'I together represent a load resistance 8.

A voltage is taken from the second secondary winding of transformer 5 and supplied through a resistance 9 to a grid III of an auxiliary tube II.

The input voltage is supplied to a grid I2 of this tube through a condenser I3 and a resistance I4. The phase-displacements owing to this condenser I3 and resistance I4 and those due to the resistance 9 and a condenser I5 provided, if required, in thecir-cuit connected to grid Ill are such that in the case of a purely ohmic character of load 8 the voltages set up at grids I0 and I2 are in phase-opposition. Usually only small phase displacement are allowed. The amplitudes of these voltages and the bias of grids I0 and I2 are chosen to be such that with the maximum permissible value of load 8 the grids just do not carry grid current.

Moreover, the ratio between the amplitudes of these voltages is such that at maximum load their control efiects on the anode current practically neutralize one another, so that the auxiliary tube either does not carry anode current or carries a constant anode current.

If the load increases excessively due to diminution of the resistance 8, which involves a decrease of the voltage induced on the second secondary winding, current pulses will occur in the anode circuit of the auxiliary tube. As a result thereof setting) and the mean anode current of the final tubes decreases again. If, however, the load becomes smaller than the maximum permissible value, due to an increase of resistance 8, the amplitude of the anode current pulses will not increase, since owing to the grid current occurring a voltage drop is produced across resistance 9, owing to which the grid 10 becomes more negative and the anode current is limited in a known manner. If required, this eiTect may be increased by providing a diode H which is connected to the rid [6.

From the displacement of the working points of final tubes l and 2 distortions will accrue, which can be obviated by controlling by means of the voltage produced across condenser !E- not the bias of the final tubes, but the mutual conductance and consequently the amplification f actor of amplifying tubes (not shown) preceding the final stage, as a result of which the effective input voltage of the final tubes decreases if the load becomes too low.

The operation of the circuit-arrangement shown in Fig. 1 will now be explained hereinafter with reference to the characteristic curves shown in Fig. 2.

In .Fig. 2 the curve a. illustrates the power of the final amplifying tubes l and 2 obtained from the anode supply with a direct voltage U, as a function of the alternating voltage Vg sup plied to the control-grid; this power is equal to USVg. Curve b represents the power S Vg R supplied to the load impedance R, curve a representing the power USVgS Vg R dissipated in tubes l and 2.

In this expression S represents the mutual conductance, Vg the alternating grid voltage of the tubes.

Since the power dissipated in the tubes should, at the most, be equal to the permissible anode dissipation Wk, i. e. Wk=WSVg-S Vg Rk, it follows that the limit value Ric of resistance R is Curve (1 in Fig. 2 represents this limit value as a function of Vg, the dotted part relating to such values of Ric which cannot be realized phys ically. At values of R smaller than Rk, i. e. the

range in the R-Vg area between curve d and the Vg-axis, the tubes are consequently overloaded.

By taking the measures according to the invention it is ensured that at any value of the load resistance the effective voltage Vg is kept beyond the range located between the Vg-axis and this curve (1.

If, in efiect, the effective alternating grid voltage is proportional, as is the case in Fig. 1, to the positive difference between a voltage C1Vg proportional to the grid voltage Vg of the final tubes and a voltage CZSVQR taken from the output, than, as a voltage Vi to be amplified and supplied to the input circuit of the final stage, Vg=ViO3(C'1VgC2SVgRC4), where C1 to C4 are constants, so that l+C C -C'2C SR i. e. as a function of R a hyperbola, represented by the dot-and-dash curve e in Fig. 1. By a proper choice of constants C1 to C4 curve a may be located as closely as possible above curve (1. A further embodiment of the invention is shown in Fig. 3, in which corresponding circuit elements bear the same reference numerals as in Fig. 1. In this circuit arrangement provision is made of two push-pull connected auxiliary tubes 9 and 9' to the grids I0 and Ill of which the voltage proportional to the output voltage, and to the grids l2 and 12' of which the voltage proportional to the input voltage of final tubes I and 2 is supplied.

The anode circuit of these tubes comprises a transformer I8, on the secondary winding of which a negative feedback voltage is induced if the voltage across the output circuit of the amplifier is too low owing to an excessively small value of load resistance 8.

The secondary winding of transformer I8 is connected in series with the secondary winding of the input transformer l9, so that the voltage supplied to the grids of tubes I, 2 is reduced by the voltage produced across the first-mentioned winding, as a result of which the effective voltage set up at the grids decreases.

It is pointed out that in the circuit-arrangement shown in Fig. 2 a decrease of the voltage supplied to the grids also occurs if the input voltage is excessively high or if it is not sinusoidal.

In the circuit-arrangement shown in Fig. 3 the input voltage Vi must not exceed a definite value. The adjustment is, however, simpler than in the circuit-arrangement shown in Fig. 1, since there is less need of taking phase-displacements into account.

In the circuit-arrangement shown in Fig. 4 the voltage proportional to the output voltage is supplied to the diode l? through a resistance 9.

Across condenser l5 a voltage is set up which influences the mutual conductance of the auxiliary tube H, by controlling the bias of grid Ill.

The voltage to be amplified, which is fed to tubes l, 2, is supplied to this grid or to another grid of the auxiliary tube II.

The voltage which is produced across condenser l6 and, similarly to the circuit-arrangement of Fig. 1, increases again according as the difference between the input and the output voltage increases, again influences e. g. the mutual conductance of amplifying tubes (not further described) preceding the final stage.

Instead of causing the output voltage to .infiuence the mutual conductance of the auxiliary tube ll according to Fig. 4, the input voltage may be used for this purpose. In this case the polarity of rectifier I! is required to be reversed.

In the circuit-arrangement shown in Fig. .4 the control is not afiected if the amplifier is loaded by a complex impedance, in contradistinction to the circuit-arrangements shown in Figs. 2 and 3, in which strong control occurs even at a small complex load. This circuit-arrangement has, however, a limitation in that the control depends to some extent upon the curve form of the incoming signal.

Fig. 5 represents, by way of example, one embodiment of the invention in which one of the voltages is operative in the cathode circuit of auxiliary tube 1 I. Otherwise the circuit-arrangement is entirely identical with that of Fig. l and has the, same reference numerals. Similarly one of the voltages may be introduced with the cathode-circuit in the circuit-arrangements shown in Figs. 3 and 4.

In the circuit-arrangements as described in the foregoing a voltage proportional to the grid voltage of the final tubes is supplied to the auxiliary tube. As an alternative, however, the

characteristic curve (1 in Fig. 2 may be closely approximated by supplying, instead of a voltage proportional to the voltage applied to the grids of tubes I 2, a voltage which is proportional 'to the output voltage of one of the amplifying tubes preceding the final tubes or proportional to the input voltage Vi of the final stage.

What I claim is: I

1. An overload protection circuit for an amplifying system having a source of desired signals, an amplifier stage adaptedior operation in class AB, B or C coupled to said source and a load impedance coupled to the output of said amplifierstage, comprising an electron discharge tube having cathode, input and output electrodes definingan input circuit and an output circuit, means coupled to said amplifier stage to derive a first potential proportional to the output voltage of said amplifier stage, means to apply said first potential to theinput circuit of said discharge tube, rectifier means coupled to the input circuit of said discharge tube to limit the amplitude of said first potential to a given value, means coupled to said source to apply a second alternating potential to the input circuit of said discharge tube, means coupled to theoutput circuit of said discharge tube to derive therefrom a control voltage at amplitude values' of said first potential below said given value, and means to vary the amplitude of said desired signals at the output of said amplifier stage proportional to the value of said control voltage;

2. An overload protection circuit for an amplifying system having an input stage providing a source of desired signals, an amplifier stage adapted for operation in class AB, B or C coupled to said input stage and a load impedance coupled to the output of said amplifier stage, comprising an electron discharge tube having cathode, input and output electrodes defining an input circuit and an output circuit; means coupled to said amplifier stage to derive a first alternating potentialproportional to the output voltage of said amplifier stage, means to apply said first alternating potential to the input circuit of said discharge tube, rectifier means coupled to the input circuit of said discharge tube to limit the amplitude of said first alternating potential to a given value, means coupled to said input stage to derive a second alternating potential proportional to the amplitude of said desired signals, means to apply said second alternating potential to the input circuit of said discharge tube, means coupled to the output circuit of said discharge tube to derive therefrom a control voltage at amplitude values of said first alternating potential below said given value, and means to vary the amplitude of said desired signals at the output of said amplifier stage proportional to the value of said control voltage.

3. An overload protection circuitfor an amplilying system having an input stage providing a source of desired signals, an amplifier stage adapted for operation in class AB, B or C coupled to said input stage and a load impedance coupled to the output of said amplifier stage, comprising an electron discharge tube having cathode, input and output electrodes defining an input circuit and an output circuit, means coupled to said amplifier stage to derive a first alternating potential proportional to the output voltage of said amplifier stage, means to apply said first alternating potential to the input circuit of said discharge'tube, rectifier means coupled to the input circuit of said discharge tube 6 to limit the amplitude of said first alternating potential to a givenvalue, means coupled to said input stage to derive a second alternating potential proportional to the amplitude of said desired signals, means to apply said second alternating potential to the input circuit of said discharge tube, means coupled to the output circuit of said discharge tube to derive therefrom a unidirectional control voltage at amplitude values of said first alternating potential below said given value, and means to vary the amplitude of said' desired signals at the output of said amplifier stage proportional to the value of said unidirectional control voltage.

4. An overload protection circuit for an amplifying system having an input stage providing a source of desired signals, an amplifier stage adapted for operation in class AB, B or C coupled to said input stage and a load impedance a coupled to the output of said amplifier stage, comprising an electron discharge tube having cathode, input and output electrodes defining an input circuit and an output circuit, means coupled to said amplifier stage to derive a first alternating potential proportional to the output voltage of said amplifier stage, means to apply said first alternating potential to the input circuit of said discharge tube in a given phase, rectifier means coupled to the input circuit of said discharge tube to limit the amplitude of said first alternating potential to a given value, means coupled to said input stage to derive a second alternating potential proportional to the amplitude of said desired signals, means to apply said second alternating potential to the input-circuit of said discharge tube in phase opposition to 'said given phase, means coupled to the output circuit of said discharge tube to derive therefrom a unidirectional control voltage at amplitudeva'lues'oi said first alternating potential below said given value, and means to vary the amplitude of said desired signals at theoutput of saidamplifier stage proportional to the-value of said unidirectional control voltage. 1 5. An overload protection circuit for an amplifying system having a source of desired signals, a push-pull amplifier stage adapted for operation in class AB, B or C coupled to said source and a load impedance coupled to the output of said amplifier stage, comprising an electron discharge tube having cathode, first control grid, second control grid and output electrodes, means coupled to said amplifier stage to derive a first alternating potential proportional to the output voltage of said amplifier stage, means to apply said first alternating potential to the first control grid of said discharge tube, rectifier means coupled to the first control grid of said discharge tube to limit the amplitude of said first alternating potential to a given value, means coupled to said source to apply a second alternating potential to the second control grid of said discharge tube, means coupled to the output electrode of said discharge tube to derive therefrom a unidirectional control voltage at amplitude values of said first alternating potential below said given value, and means to vary the amplitude of said desired signals at the output of said amplifier stage proportional to the value of said control voltage.

6. An overload protection circuit for an amplifying system having a source of desired signals, a push-pull amplifier stage adapted for operation in class AB, B or C coupled to said source and a load impedance coupled to the output of said amplifier stage, comprising a pair of electron discharge tubes coupled in push-pull and each having cathode, first control grid, second control grid and output electrodes, means coupled to said amplifier stage to derive a first alternating potential proportional to the output voltage of said amplifier stage, means to apply said first alternating potential to the first control grid of each of said discharge tubes, rectifier means coupled to the first control grids of said discharge tubes to limit the amplitude of said first alternating potential to a given value, means coupled to said source to apply a second alternating potential to the second control grids of said discharge tubes, means coupled to the output electrodes of said discharge tubes to derive therefrom a control voltage at amplitude values of said first alternating potential below said given value, and means to vary the amplitude of said desired signals at the output of said amplifier stage proportional to the value of said control voltage.

7 An overload protection circuit for an amplifying system having a source of desired signals, a push-pull amplifier stage adapted for operation in class AB, B or C coupled to said source and a load impedance coupled to the output of said amplifier stage, comprising an electron discharge tube having cathode, input and output electrodes, means coupled to said amplifier stage to derive a first alternating potential proportional to the output voltage of said amplifier stage, rectifying means coupled to the input electrode of said discharge tube, means to apply said first alternating potential to said rectifying means to limit the amplitude of said first alternating potential to a given value and to apply a bias voltage proportional to the amplitude of said first alternating potential to the input electrode of said discharge tube, means coupled to said source to apply a second alternating potential to the input electrode of said discharge tube, means coupled to the output electrode of said discharge tube to derive therefrom a unidirectional control voltage at amplitude values of said first alternating potential below said given value, and means to vary the amplitude of said desired signals at the output of said amplifier stage proportional to the value of said control voltage.

8. An overload protection circuit for an amplifying system having a source of desired signals, a push-pull amplifier stage adapted for opera-.- tion in class AB, B or C coupled to said source and a load impedance coupled to the output of said amplifier stage, comprising an electron .discharge tube having cathode and input electrodes defining a cathode-input electrode circuit and having an output electrode, means coupled to said amplifier stage to derive a first alternating r potential proportional to the output voltage of said amplifier stage, means to inject said first alternating potential into the cathode-input elec, trode circuit of said discharge tube, rectifier means coupled to said cathode-input electrode circuit to limit the amplitude of said first alternating potential to a given value, means coupled to said source to apply a second alternating potential to the input electrode of said discharge tube, means coupled to the output electrode of said discharge tube to derive therefrom a unidirectional control voltage at amplitude values of said first alternating potential below said giv en value, and means to vary the amplitude of said desired signals at the output of said amplifier stage proportional to the value of said con trol voltage.

PIETER KLAAS BUYS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Hammond, Jr. Dec. 31, 1946 

